In
most systems from small ponds to the oceans bacteria have the largest
collective biomass, most numbers of organisms, and play key roles in all
areas from production to consumption, including interchange of electrons
during metabolism.They are in
virtually all habitats, from free-living to attached, external and internal
to all organisms, and their metabolites are important whether positive products
or lethal toxins.

Heterotrophic bacteria, both active and inactive,
seem to be present in most freshwater lakes at concentrations in the range of
105 to 107 ml-1 (Kuznetsov 1970).Samples of unfiltered
(“whole”) lake water sampled at close intervals (1 – 10 cm
depth intervals) often vary greatly in bacterial concentration, with
‘microstratification’ (Baker 1973) evident in water density
gradients, usually within the thermocline or metalimnion.The heterotrophic communities are also
associated with autotrophic plankton, either in precisely the same layers
(‘plates’ or ‘blankets’), or in nearby strata.

Iron oxidizing bacteria.

A variety of chemoautolithotrophic bacteria capable of oxidizing
iron- or sulphur-containing minerals can be isolated from acidic mine
drainage or ore exposed to water and the atmosphere. The mineral-oxidizing
bacteria found in these conditions at ambient temperatures are ubiquitous,
the most commonly encountered characterized as Thiobacillus ferrooxidans, `Leptospirillum ferrooxidans',
Thiobacillus thiooxidans and more recently Thiobacilluscaldus. T. ferrooxidans (iron and sulphur
oxidizer) and `L. ferrooxidans'
(iron oxidizer) can oxidize ore such as pyrite when growing in pure culture. T. thiooxidans and T. caldus are both sulphur oxidizers
that cannotoxidize pyrite alone
but grow on the sulphur released after the iron has been oxidized. Also found
in similar environments is a variety of acidophilic heterotrophs or
facultative heterotrophs in the genera Acidiphilium,
Acidocella and `Ferromicrobium'.Oxidation of ore by consortia of
bacteria generally takes place at a higher rate than with pure cultures (Rawlings et al. 1999).

Photosynthetic
bacteria

At
present (2015) seven distinct groups of photosynthetic bacteria are
recognized.All but one contain
bacteriochlorophyll (Bchl), one or more of Bchl a through Bchl g, and
are anoxygenic.The only oxygenic
group has chlorophyll (Chl) instead of Bchl, and is oxygenic (Cyanobacteria).

Green sulfur bacteria,
e.g. Chlorobium and Pelodictyon, are strict anaerobes
found only in anoxic habitats such as the hypolimnia of mesotrophic to eutrophic
lakes, and subsurface layers of salt marsh mats.

Purple
sulfur bacteria such as Chromatium
are restricted to low concentrations of dissolved oxygen (microaerobes) and
anoxic habitats.They often lie
above the green sulfurs in microlayers of lakes and salt marsh mats.

Purple
nonsulfur bacteria such as Rhodospirillum,
Rhodobacter and several others are
diverse in their nutrition and habitatsThey are mainly anaerobic photoheterotrophs that survive under at low
light intensity with several organic substrates.Some ferment.Many are also photoautotrophs or
chemoheterotrophs that grow in microaerobic or even aerobic conditions.Some species switch modes of
metabolism das a result of habitat conditions.One of another species occupies
various diverse habitats include freshwater and marine aquatic habitats, also
soils, sewage lagoons and plants.

Acidobacteria,
first recognized in 1997, are soil dwellers that was described in 2007.Some are acidophlic such as Choroacidobacterium.

Heliobacteria
were discovered by a field class at Indiana University in 1981 given the name
Heliobacterium chlorum.They live in soils and contain Bchl g, and lack intracytoplasmic
memberanes so that all their pigments are in the cytoplasmic membrane.

Green and
red filamentous bacteria, formerly known as green non-sulfurs, are
anaerobes that can be photoautotrophs, chemoorganotrophs or
photoheterotrophs.

Cyanobacteria
are the first known oxygenic photosynthesizers and contain Chl a along with
two water-soluble ‘phycobilin’ pigments’ = phycocyanin, a
blue pigment, and phycoerithrin, a red pigment.the phycobilins absorb in the light
spectrum where chlorophyll a is least efficient, thus increase the light
harvesting ability of cyanobacteria at low light intensity.Many if not all cyanobacteria are
capable of switching between anoxygenic and oxygenic photosynthesis.

References

Baker, A.L.1973.Microstratification of phytoplankton
in selected Minnesota lakes.PhD
Dissertation, University of Minnesota.

Kuznetsov, S.I. 1970. The
Microflora of Lakes and its Geochemical Activity. [English translation by the
Israel Program for Scientific Translations, Jerusalem, Israel;C.H. Oppenheimer, Ed.] University of
Texas Press (503 pp).

Rawlings, D.E.,H. Tributsch and G. S. Hansford1999.Reasons why ‘Leptospirillum
’-like species rather than Thiobacillus ferrooxidansare the
dominant iron-oxidizing bacteria in many commercial processes for the
biooxidation of pyrite and related ores.Microbiology 145:5-13.